The invention relates to a process for the preparation of zinc dithiocarbamates, wherein the zinc dithiocarbamates are obtained in especially readily filterable form and high purity.
The preparation of zinc dithiocarbamates, which are used primarily in plant protection as fungicides and in the rubber industry as vulcanization accelerators for natural and synthetic rubbers, is conventionally carried out by reacting secondary amines with carbon disulfide in the presence of ammonia or lye to form the corresponding dithiocarbamates, which are then converted by means of zinc salts into the sparingly soluble zinc dithiocarbamates.
A further possible method of preparing zinc dithiocarbamates is the reaction of secondary amines in the presence of zinc oxide and carbon disulfide.
A disadvantage of the known processes for the preparation of zinc dithiocarbamates is especially that the zinc dithiocarbamates obtained are of small particle size and irregular particle-size distribution, and accordingly, are difficult to filter. Since the particle sizes obtained are greatly dependent on the particular precipitation temperature of the zinc dithiocarbamates from the reaction mixturexe2x80x94a larger particle size at a higher precipitation temperaturexe2x80x94one possible method of obtaining larger particle sizes would be to raise the precipitation temperature. That possibility is limited, however, by the fact that the carbon disulfide used has a low boiling point and therefore, is readily volatile. Furthermore, carbon disulfide is readily combustible. For that reason, precipitation of zinc dithiocarbamates at a higher temperature is problematic.
The object of the present invention is to provide a simple process for the preparation of zinc dithiocarbamates which, in particular, yields zinc dithiocarbamates in readily filterable form, that is to say with a large particle size and a distribution of the re-suiting particle sizes that is as uniform as possible.
Accordingly, the invention provides a process for the preparation of zinc dithiocarbamates of the formula 
wherein
R1 and R2 are identical or different, preferably identical, and represent C1-C6-alkyl or C7-C12-aralkyl radicals, or wherein R1 and R2 are bridged by polymethylene chains having from 2 to 12 methylene segments, it being possible for the polymethylene chains to be interrupted by from 1 to 3 hetero atoms, such as oxygen, sulfur or nitrogen,
wherein the process is characterized in that zinc salts are first added, in aqueous solution, to the secondary amines underlying the zinc carbamates, so that a zinc salt complex of the amines is formed, and carbon disulfide and sodium hydroxide solution are then introduced, with stirring, into the aqueous suspension so obtained, the temperatures being maintained at from room temperature (approximately 20xc2x0 C.) to the boiling temperature of carbon disulfide (42xc2x0 C. at normal pressure) until the reaction has completely finished.
The process according to the invention can be illustrated using the example of the preparation of zinc dibenzyldithiocarbamate by the following reaction scheme: 
Suitable secondary amines that can be used for the preparation of the dithiocarbamates are, for example: dimethylamine, diethylamine, dipropylamine, dibutylamine, piperidine, morpholine, N-methylpiperazine, N-ethylpiperazine, dibenzylamine, diarylamine, ethylaniline, methylaniline, methyl-tert.-butylamine and ethyl-tert.-butylamine.
Dibenzylamine, dimethylamine, diethylamine, dibutylamine, piperidine and morpholine are preferred, and dibenzylamine is most preferred.
Examples of zinc salts that are reacted according to the present invention with the secondary amines in aqueous solution, preferably under an inert gas atmosphere (e.g. nitrogen), are: zinc sulfate, zinc chloride and/or zinc acetate, in particular zinc sulfate.
The zinc salts are preferably reacted with the secondary amines in from 1 to 25% aqueous solution, the zinc salts being used in a ratio of from (0.8 to 1.2):1, preferably 1:1, relative to the secondary amines.
The zinc salt complexes of the secondary amines so obtained are then reacted with carbon disulfide, the amount of carbon disulfide used being from a slight excess up to a maximum of 5 wt. %, based on the amount of zinc salt complexes of the secondary amines.
The reaction of the zinc salt complexes of the secondary amines with carbon disulfide takes place with vigorous stirring, the suspension preferably being maintained at from 30 to 40xc2x0 C.
With the addition of lye, preferably sodium hydroxide solution (from 5 to 10% strength), stirring is carried out, at the temperature indicated, until the reaction has completely finished. The sodium hydroxide solution is normally employed in a stoichiometric amount. A slight excess of up to approximately 5 wt. % is possible.
The resulting zinc dithiocarbamate is then filtered, washed, for example with water and/or with isopropanol, and then dried, advantageously in vacuo.
The zinc dithiocarbamates are obtained by the process according to the present invention with average primary particle sizes, which are a multiple of the normal size, the particle-size distribution being very homogeneous. In the case of zinc dibenzyldithiocarbamate, the primary particle size obtained according to the present invention is approximately 100xcexc instead of 5xcexc as in the synthesis method employed hitherto, as will be seen with reference to the SEM image. The yields by the process according to the present invention can reach over 99% of the theoretical yield. In general, they are from 98 to 99%.
In the process according to the present invention, it has proven advantageous, particularly in the case of longer-chained amines, to carry out the reaction of the secondary amines in aqueous solution with the zinc salts in the presence of an emulsifier, such as Emulgator(copyright) L3 from Bayer AG. The amount of emulsifier is approximately from 0.1 to 0.5 wt. %, based on secondary amine used, but preferably from 0.2 to 0.3 wt. %.